It is less than futile, in fact, it is outright nonsensical, to argue in favour of or promote an explanation that is in contradiction with observational evidence. What is required is to find, or at least try to find, a sound and well-founded explanation. And not just for some of the observations, but for each individual observation, as well as for the entire ensemble of observations. This is what we should do.

Fasting means not eating; everyone knows that. The meaning of the word has been loosened to include not consuming appreciable amounts of calories, as in doing a green juice fast, for example, but which should instead rightly be called a cleanse. The expression intermittent fasting implies a cycle of some kind, and is used to mean not eating for periods of 16, 18, 24 or 48 hours, but on a regular basis, like every week or even every day.

Fasting has been known and recognised for its often quasi-miraculous curative effects for thousands of years. Indeed, it is possible to find accounts of individuals recovering from just about any ailment and disease imaginable simply from fasting long enough. It seems, however, that fasting as a healing modality, has, over the past couple of centuries, steadily grown less popular in the medical profession and, as a consequence, also in the general population.

A resurgence of scientific interest over the last decades in the benefits of fasting for treating various degenerative conditions like arthritis and cancer, but also for extending healthy lifespan about which I will write at one point in the future, has brought it back into the spotlight, especially in circles of optimal health enthusiasts, which includes some gym go-ers and body builders interested not so much in optimal health, but mostly in losing fat and gaining muscle.

Therefore, there has been quite a few people trying out or adopting intermittent fasting for periods of a few weeks to a few months, or even longer, but reading things here and there shows that they have had varying success given their initial motivations, whatever those might have been.

Ori Hofmekler was one of the first to popularise the idea of intermittent fasting with his book The Warrior Diet. He has continued to write and to encourage intermittent fasting for a wide range of benefits, especially in regards to the goal of improving body composition, as one of his last titles expresses perfectly: Maximum Muscle, Minimum Fat.

Dr Hertoghe, the world famous endocrinologist and anti-ageing specialist, as well as Mark Sisson (Primal Blueprint) have also been vocal and influential proponents of intermittent fasting for a while. More recently, Dr Mercola did several interviews with Hofmekler, and wrote a few articles on the topic, sharing his experience and enthusiasm for the health and fitness benefits intermittent fasting can bring. These are just some of the well known players that I know of and respect in the natural health community, that have endorsed and promoted this kind of cyclical fasting.

Naturally, as is the case for almost any topic we can think of, there are opposing opinions and, in fact, bashing of intermittent fasting as a means to improve health and body composition, especially in the popular fitness and gym culture. And, as is also the case for almost any topic we can think of, contradictory views and opinions are usually caused by misunderstanding, or at least, incomplete understanding of the elements involved, and in particular the more subtle ones.

On the one hand, we have the proponents claiming that we can very effectively get much healthier, with much improved energy levels, mood, digestion, and natural detoxification and excretion of metabolic acids; normalise and recover the optimal balance of specific hormones, and eventually, of the entire hormonal system; over time lose all excess body fat reserve, increase flexibility and hasten recovery, better preserve our precious muscle tissue and build more very efficiently. And these are just some of the claimed (but also documented) benefits of intermittent fasting.

On the other hand, the nay-sayers and bashers report that these claims are more than just false, they are, in fact, often the exact opposite of what they have found or seen for themselves or in others coming to them for help and expert advice. Reports of feeling really terrible, with massive headaches, bad digestion, awfully low energy levels, and thus, obviously, very bad and destructive moods; loss of some fat but also, over time, of lots or maybe even most of their muscle tissue; extreme hunger, with frightening ravenousness when evening mealtime comes around, leading to monstrous, uncontrolled and uncontrollable overeating without discrimination of food kinds or quality, and over time, showing obvious signs that can be identified as those associated with eating disorders.

How is it possible to have research, studies and documented cases—plenty of documented cases—that provide observational evidence—proof, if you prefer—that support the claims of both of these camps? How can we observe and actually measure such profoundly different consequences in different people that are supposed to follow comparable diets, consequences that are diametrically opposed to one another. In other words, observational evidence that appears to be completely and totally contradictory?

A simple approach, the one espoused by many, maybe most, of the intermittent fasting bashers, is to just say that proponents are wrong and imagining things, letting themselves be fooled by the hype, but actually blind to the reality of the detrimental consequences of practicing cyclical fasting.

For me, the only satisfactory approach is the one that seeks to explain all the observations, to reconcile all the observational evidence, and make sense of the entire ensemble of information available through a physiology and biochemistry based explanation that is complete. I also think it is fair to say that there are more better informed proponents than there are opponents, but this is not obviously the case, and I would thus not bet much on this claim.

Here it is, the crux of the matter, the one single crucial element needed to understand and explain the wide spectrum of apparently contradictory observations that is overlooked because it is misunderstood:

The body’s response to intermittent fasting is entirely dependent upon the state of one’s metabolism, and everything about it hinges on the physiology of nutritional ketosis. 

In fact, the vast majority of the benefits of intermittent fasting are those derived from nutritional ketosis but heightened by the fasted state, and therefore, can only become manifest if the fasting individual is keto-adapted and remains in nutritional ketosis most of the time.

You might be thinking: what in the world is nutritional ketosis, and where’s the explanation for the contradictory observations? Nutritional ketosis is the metabolic state in which the liver manufactures ketone bodies from fat to provide fuel for the brain cells that can only use glucose or ketones for their energy needs. This only happens if and when circulating insulin levels are low, and when blood glucose stays below 80-90 mg/dL for a period of 24-48 hours (generally speaking, on average, and in normal circumstance). The reason is fat will not be burned for fuel is there is plenty of glucose in the blood, and in order to burn fat, insulin must be low.

This metabolic state is induced either by fasting—this is the quickest but also most extreme way to do it, or by eliminating insulin-stimulating carbohydrates (sugars and starches) from the diet—this is by far the easier and obviously much more sustainable way to do it. The longer it is maintained, the better adapted the metabolism becomes. But before ketones are produced to fuel the brain, the body goes through metabolic changes to which it tries to adapt as best it can. The most important but also most severe of them all, is the fundamental shift from using glucose as the primary fuel, not just for the brain, but for all cellular energy needs in the body, to using fats, both from body fat reserves and from food.

The bane of our time is global, chronically elevated insulin levels. Hyper-insulinemia, as it is technically called, sits squarely as one of the root cause of all the diseases of civilisation that kill most (90%) of us today, more or less uniformly across the planet. What does this have to do with our considerations of intermittent fasting? It has everything to do with it:

Insulin is the master hormone that orchestrates the metabolism in what relates to storage and usage of macronutrient (carbs, fats, and proteins) at the cellular level.

Chronically elevated insulin always and inevitably leads to insulin resistance. Insulin resistance means that cells do not respond to insulin as they should, and require ever increasing concentrations of insulin in order to move glucose into the cell. And ever increasing concentrations of insulin means ever increasing inability to use fat cellular fuel, with particular difficulty in unlocking and tapping into the usually greatly overabundant reserves of body fat.

What is truly remarkable is that insulin resistance, even if it has been developing and growing steadily with each passing day and with each high carb meal or snack over our entire lifetime, it can be reversed in weeks when insulin-stimulating carbs are eliminated from the diet: 48 hours to enter nutritional ketosis; one week for water retention release, initial intestinal detox and basic adaptation to fat-burning; four weeks for functional keto-adaptation; and 8 weeks for complete keto-adaptation.

Eliminating insulin-stimulating carbs eliminated the need for large insulin secretions by the pancreas. Therefore, both glucose and insulin concentrations steadily decrease with time, and eventually fat-burning and ketone production kicks in, marking the first step in the transition of the metabolism from sugar-burning to fat-burning, which is what we referred to as fat- or keto-adaptation.

There is a catch though: before fat-burning and ketone production begins, the metabolism of the insulin resistant individual will go through withdrawal from its sugar addiction. First, sugar levels start to drop. After a number of hours, 3 to 4 hours say, blood sugar is too low to supply enough fast-burning glucose to cells for their metabolic activities. Because insulin remains high, and because the body is highly insulin resistant, as we said, it is not possible to use fat from the body’s fat stores. Therefore, it is the liver that comes to the rescue and begins to convert its stores of glycogen into glucose and pumping that into the bloodstream to provide cellular fuel.

Within a few hours, however, the glycogen in the liver is depleted, and blood sugar drops once again, and lower still. Because the body remains unable to tap into its fat reserves due to the state of insulin resistance, it has, at this point, no choice but to turn to muscle tissue, from which it is far easier to breakdown protein and manufacture glucose than it is to start burning fat. And thus, the muscles are eaten away in order to provide the glucose to all of the multitude of insulin resistant (sugar-addicted) cells throughout the organism.

We now come to the final analysis of our observational evidence in regards to intermittent fasting, and consider two scenarios that can explain, as it rightly should, the ensemble of observations in its entirety, and thus clarify and reconcile the apparent contradictions that are seen, and which lead to serious confusion about the issue, even, and maybe especially, among our health, fitness and bodybuilding experts.

Scenario 1: We take a perfectly keto-adapted person who has been eating a diet devoid of insulin-stimulating carbs for a long time, and who therefore always has very low glucose and insulin levels, and as a consequence, exquisite insulin-sensitivity. What happens if they stop eating? Nothing special, really. Their body is always using fat and ketones to supply all healthy body and brain cells with their metabolic energy needs. So, if there is no fat that is provided through the digestive system, then it is taken, without any trouble or noticeable changes in energy levels or concentration, from the body’s fat reserves that are always plentiful, even in the leanest among us with single digit body fat, because 1 gram provides 9 calories, which means that we need only about 200 g for a whole day of normal activities, and have at least 5 kg at any given time (8.5% fat on 60 kg body weight).

Moreover, if we exercise during the fast, there is no noticeable difference because at low intensity, cellular energy needs are taken care of by fat which is continuously released from the fat stores into the bloodstream, while at higher intensity the glycogen stored in the muscle cells themselves, can be used in the form of quick burning glucose together with additional supply from the liver than converts its stores of glycogen if need be (if stress hormones are secreted).

So, biking and working out with weights, for example, is perfectly fine and actually feels great. Even more interesting is the fact that stimulating the muscular system by exercising while fasting triggers the release of various hormones in addition to growth hormone for which there is nothing more effective than fasting, whose purpose is primarily to preserve those physiologically important muscle tissues as essential for functional survival, while breaking down to recycle the proteins of other tissues which are not required like lumps, tumours, and scar tissue. And this means that the hormonal environment created by exercise under fasting conditions is conducive to both preserving and building more muscle, all the while also expediting and maximising fat-burning. And this is what is observed.

Hunger is present at times, but is certainly far from being problematic. There are no headaches, no stomach pains, no sleepiness, no scattered mental discursiveness, no problems concentrating or working. Sitting down to eat the evening’s nutrient-dense, enzyme-rich and high fat meal with adequate amounts of protein for tissue repair and muscle building, is nourishing, perfectly satisfying, and well digested throughout the evening and night, as long as we eat several hours before going to bed. No over-eating, no cravings, no psychological disturbances, no problems at all. A picture of perfect metabolic efficiency.

Scenario 2: We take an average but pretty active person from the general population who eats a standard diet with plenty of insulin-stimulating carbs, both simple sugars, and complex carbs in the form of pasta, rice, whole grain bread, etc (70% of calories), and who therefore always has high blood glucose and insulin levels, and as a consequence, pretty strong insulin resistance. What happens if they stop eating? We saw this earlier: blood glucose drops, but not insulin; the liver starts to pump out glucose to pick up the slack, and runs out after about 3-5 hours; sugar drops once more, but not really the insulin; since fat stores cannot be tapped into, muscle tissue is broken down to manufacture glucose; longer period of fasting means more muscle breakdown.

If we exercise gently, things are fine at first because we can tap into the glycogen stored in the muscles, but will soon get much worse because we increase the energy demands, but continue to be unable to use body fat stores, and therefore increase the rate at which muscle tissue is broken down, especially if we do weights and high intensity training.

Low intensity aerobic exercise depletes glycogen from the muscles and when it runs out, we feel exhausted, completely flat out. (This is the same as hitting “the wall” in long distance events, and only occurs because the body cannot readily tap into its fat reserves: a well keto-adapted athlete never really hits any such walls!) Far worse is high intensity exercise, which causes more intense and faster muscle breakdown, the higher the intensity, the more muscle breakdown.

Waking up in the morning after a night’s sleep (and unconscious fast), we are starving, dearly longing for the bread, the jams, the cereals, the orange juice, the waffles, the maple syrup, and everything else we can imagine, but we hold out and go to work. Every hour is excruciating, terrible headache, hunger pains throughout the abdominal cavity, but when these subside, we are falling asleep, with a complete inability to concentrate on anything at all. We feel like shit.

By the time evening rolls around, we are so ravenous we would eat a horse. So we sit down and eat, and eat, and eat everything we can get our hands on: pizza, pasta with sauce and cheese, garlic bread with butter, steak and potatoes or french fries, and then desert, sweets, oh man, we waited all day to eat, and now we can eat anything and everything we want, because tomorrow we’ll be starving again for the whole day. We get up in the morning, and the whole cycle starts over again.

Over time we kind of get used to it, but because we don’t understand the most essential element of the whole thing—nurturing nutritional ketosis—we remain just as insulin-resistant, every day we feel shitty, every night we eat like a pig, and throughout the whole time, more or less, we break down muscle, and our insulin resistance prevents appreciable fat loss. After doing this for a while and seeing the detrimental effects of this regime, we go seek help from a fitness expert. They tell us that this intermittent fasting thing is a load of shit, and as them, grow instantly convinced that all the stuff people say about the benefits it can bring for optimal health and improved body composition is also a load of shit: if it didn’t work for me, then it simply cannot work for anyone.

Unfortunately, neither we nor the fitness expert understands enough physiology, biochemistry, and endocrinology to be able to make sense of these conflicting and contradicting accounts, personal experiences, and observations reported in the scientific literature, and just settle into this view that it really is a load of BS, and that it might work a little, sometimes, on some people, but not on others, and no matter what, it always leads to pathological states of mind, if not full fledged eating disorders.

It is my hope, however, that you are now able to see how these very observations, as conflicting, contradictory, and certainly quite puzzling as they may seem at first, can be explained and reconciled marvellously well in light of a better understanding of the basic principles of energy metabolism, and of the remarkable but unfortunately almost universally misunderstood state of nutritional ketosis, that most medical professionals usually mistake for the pathological condition of diabetic ketoacidosis.

Finally, in closing, I have a confession to make: I have been experimenting with intermittent fasting in one form or another for many years now. I never eat anything before midday, and on most days until about 14:00, which makes it an approximately 18-hour fast from 20:00 the night before. On weekends, I fast until noon, and then go do weight training. On those days, I usually eat for the first time around 17:30, and make that my single meal of the day. On some days I eat a large lunch and dinner to increase my overall calorie and protein intake. I usually workout 3-4 times a week, and usually in the late afternoon-early evening.

I have not experienced loss of muscle since I dropped the insulin-stimulating carbs from my diet in 2007. Both muscle tone and strength is maintained very well even after long periods without resistance training. I have, however, never made a particular effort to gain muscle mass. This year, I would like to see how much muscle I can put on, and will thus put the science to the test for myself. If you are interested, don’t worry, I’ll keep you posted. If you’re not, then that’s fine too.

But if there is a single thing you must remember from what I wrote, it is this: you can only really benefit from intermittent fasting when you are keto-adapted, and remain in a state of nutritional ketosis the majority of the time. Otherwise potential benefits are lost, and the practice can become rather detrimental.

How long do you think these hunters hunt each day? Do you think they have a big breakfast before going, or a large lunch while they are out? How long do you think they are out before they settle back around the fire in their village to have their main meal of the day? And what do you think they will eat when they do return with their catch of the day?

(This article was written after reading this article by Dani Shugart on T-Nation sent to me by a friend who knew I would have some remarks to make, and probably some clarifications to bring to it.)